Low Rolling Resistance Tires
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Low rolling resistance tires are tires which are designed to improve the fuel efficiency of a vehicle (particularly hybrids) by minimizing the energy wasted as heat when the tire rolls down the road. A low-rolling-resistance tire creates less friction during operation than a conventional tire, and, because friction between the road surface and the tire requires extra power to overcome, the net result is better fuel economy versus the same vehicle equipped with conventional tires. This increased fuel efficiency contributes to cleaner-running vehicles, because, as a rule, vehicles that use less fuel also produce fewer emissions. Most hybrid vehicles use some sort of low rolling resistance tires.
Rolling resistance, sometimes called 'rolling friction', is the resistance that occurs when an object (e.g a wheel or tire) rolls. It is caused by the deformation of the wheel or tire or the deformation of the ground. It depends very much on the material of the wheel or tire and the sort of ground. For example, rubber will give a bigger rolling friction than steel. Also, sand on the ground will give more rolling friction than concrete. A vehicle rolling will gradually slow down due to rolling friction, but a train running on a steel rail will roll much further than a car or truck on rubber tires.
Several factors affect the magnitude of rolling friction a tire generates:
- Material - Tires with higher sulfur content tend to have a lower rolling friction. This is one strategy that most hybrid car vendors use to improve fuel efficiency. The general rule is that harder tires equal less rolling resistance; however, tires that are too hard have a smaller contact patch with the road and will affect the vehicle's handling.
- Dimensions - Rolling friction is proportional to the contact area of the tire - so a thinner tire will exhibit less friction (but also less grip) than a wider one.
- Extent of inflation - Partially inflated tires tend to suffer higher rolling friction, and may lead to overheating. For example - a vehicle that requires its tires to be inflated to 35 psi will have an increase in rolling resistance of approximately 12.5% if the tires are allowed to become underinflated to just 28 psi.
Statistics
- 80% or more of a car’s fuel energy is wasted by friction and other such losses.
- 1.5 to 4.5% of total gasoline use could be saved if all replacement tires in use had low rolling resistance.
- About 237 million replacement tires are sold in the U.S. each year – none have rolling resistance labeling.
